Cracks, damaged areas and other similar defects in jet engine and gas turbine components, such as but not limited to turbine buckets, blades and vanes, are of course extremely undesirable and dangerous if larger than permitted by design practice and standards. The cracks, damaged areas and other similar defects are formed during service, and are due, in part, to effects of one or more of mechanical fatigue, thermal fatigue, creep rupture, and foreign object damage. Additionally, significant metal loss can occur by oxidation or corrosion in an engine's environment.
The cracks, damaged areas and other similar defects can be localized in an area of an engine component. The remainder of the area of an engine component may be subjected to less severe stress and thermal environments, however these would not jeopardize performance of the engine component for many hours of continued service. In this situation, the repair of the engine component may have a significant economic savings and value. The repair of the engine component need not necessarily be to original performance levels, but need only be to a predetermined acceptable level of those original performance levels.
Currently, in order to repair engine components, damaged areas of the engine components are treated to clean any oxide from both external airfoil surfaces and internal faces. Then any cracks, damaged areas, and other such defects are filled in with powder mixes, which are at least partially melted in a repair braze thermal cycle. The powder mixes include at least one low-melting powder composition, where the powder mixes will melt and flow, for example by capillary action. Thus, melted flowing powder mixes will fill deep cracks, carrying some higher-melting, still-solid, powders along with the flowing melt.
During the repair cycle, at least a partial dissolution of the higher-melting powders and substrate surfaces occurs. The partially dissolutionized higher-melting powders and substrate surfaces will flow into the lower-melting liquid. This flow will continue until the liquid composition is altered, thus a melting range of the diluted melted liquid composition is increased, and freezing of the liquid composition occurs.
The engine components often are formed from a composition comprising a Ni-base alloy composition. Accordingly, to achieve a satisfactory flow in a low-melting alloy, various amounts of melting point depressants, such as Si and B, are commonly used in a Ni-base braze repair alloy composition. These levels of melting point depressants can reach levels up to about 6 atomic percent (a/o) Si and greater than about 12 a/o B. These levels of melting point depressants, such as Si and B, insure wetting of the solid surfaces of the engine components, and significantly reduce a melting range of the Ni-base braze repair alloy composition, which contains the Si and B as melting point depressants.
However, these melting point depressant levels, such as Si and B, make repeated repair procedures risky, because large portions of multiply repaired airfoils are often subject to melting in hot streak transients. The hot streak transients often occur in engine components during use. These levels of melting point depressants, such as Si and B, can also lead to a decreased rupture life, since rupture is effected by the proximity of a service temperature to an incipient melting range, for the compositions of the alloys in the engine component and the repair alloy composition.
Further, both B and Si constituents in a Ni-base braze repair alloy composition can lead to large fractions of the repaired engine component regions being converted to undesirable and often detrimental brittle intermetallics and intermediate phases. The brittle intermetallics and intermediate phases also comprise undesirable silicides and borides, which also are detrimental to engine components, especially when repaired.